October 11, 2022

An introduction to heating and cutting gems in Sri Lanka

Day four in Sri Lanka is an educational insight into two lesser known processes along a gemstones' journey.

We wake up to a clear sky but the air is heavy and thick, as if a downpour is imminent. Considering the rain, we quickly pack up the van and set off, retracing the now well-trodden path to Batugedara.

We arrive at our destination just as the sun starts to blaze through the clouds. Our location is a house; it's small and shrouded by dense foliage and inside, there's a man sat in the dim obscurity of this cave-like den. Glowing amid the darkness is a small brick kiln, with white hot coals that crackle as they let off pockets of trapped steam. The man doesn't speak English but we communicate through Armil and our best hand signals and nods.

An introduction to gemstone heat treatment

Extract from our Sri Lanka film, view the full film here.

The process of heating gemstones has been practised for centuries and is an accepted practice in the gemological trade. When very light coloured gems are exposed to extreme heat it alters the colour and increases the stone's clarity.

Taking sapphires as an example, these stones typically crystallise in temperatures between 700-1300 degrees Celsius. When formed at the upper end of this heat range generally more saturated and cleaner gemstones are created.

As part of "heat treatment" rough (uncut) gemstones are heated at intense temperatures to mimic the conditions within the earths mantle. This alters the stone's structure causing light to enter and exit the gem differently, and ultimately enhancing the colour.

Sometimes gemstones can change colour completely as part of the heating process. For example Tanzanite, which changes from a dull earthy-brown to a brilliant purplish blue. Other gemstone colour changes can be more subtle, from a sky-blue to a royal-blue or from yellow to white.
 

Every stone reacts differently at and therefore the art of heating is knowing when a stone may benefit from such a process.

In traditional methods of heating, rough gems are placed into a small clay kiln. Coconut husks are used to fuel the fire, which can be intensified by using a blow pipe. This process isn’t an exact science, but a lot of skill is required to know how the stone will behave once it's exposed to such high temperatures.

Modernised methods of gemstone treatment use electrically operated kilns, where the temperature and time exposed to heat can be controlled more precisely. In both these methods the resulting colour of the gemstone, depends on the length of time and the temperature of the kiln.

At what stage does heat treatment take place?

Generally, heat treatment is carried out before the gems are cut. This is because during the heating process any inclusions or fissures – formed during the crystallisation process in the earth – can expand, and cause the stone to crack. Sadly these naturally formed imperfections can't be avoided. 
 

Gemstones that display naturally saturated colours and therefore don't require heat treatment must be truly exceptional. Only approximately 5-10% of sapphires you see in fine jewellery are not heat treated. 
 

How does heat treatment impact a sapphire's clarity?

As we've already pointed out, many sapphires have inclusions which are formed of very fine hair-like crystals of a mineral called rutile. Rutile is a titanium oxide and often it forms with a sapphire crystal, becoming embedded within the structure of the crystal.

The threads of rutile typically form along the crystal axes, following the hexagonal outline of the crystal. Too much rutile in a sapphire can negatively impact the stone's appearance, making it look dull and lifeless. Using heat treatment dissolves any appearance of rutile, improving the clarity and enriching the colour of the sapphire.

A sapphire's melting point is 2,044 degrees Celsius, meanwhile, rutile melts at 1,843 degrees Celsius. Heat treatment takes advantage of this difference by heating the sapphire to a high enough temperature to dissolve the rutile, without melting or damaging the sapphire.

When the sapphire is heated and then held at a high temperature for a number of hours, the rutile dissolves, improving the clarity of the gem. 
 

Why does heating improve a sapphire's colour?

The rich blue colour you typically see in sapphires is created through a combination of titanium and iron that's trapped in the stone. Tiny amounts of each mineral collide in a way that produces a beautiful blue colour in the gem. The process at work is called intervalence charge transfer or cooperative charge transfer, which is the transfer of an electron from one transition-metal ion to another.

The heating causes the titanium of the rutile crystal to diffuse into the sapphire crystal, and in the presence of iron, a beautiful blue colour develops. Heat can also change the colours created by other minerals as well. For example, high enough iron content will give a green or yellow colour to a sapphire.

The use of heat treatment and other enhancement practices

Heat treatment isn't the only practice used to enhance the appearance of a gemstone. There's oiling; a process of filling stones (typically emeralds) with oil to improve their colour and reduce the appearance of flaws; irradiation (commonly used on tourmaline) which uses radiation to alter a stone's colour, and many more techniques that all help to give the most durable, vibrant stone.

Heat-treatment is widely accepted in the fine jewellery industry because it mimics a natural process and therefore doesn't damage the integrity of the stone, and it doesn't require the use of chemicals.

The key thing to remember is that the majority of gemstones on the market, and in particular rubies and sapphires, have had some form of heat treatment to ensure you are getting the very best stone on the market. 
 

An overview of gem cutting and polishing

The cutting of gemstones, or lapidary, is an art form. It takes a lot of skill, cultivated over years of fine tuning and to see lapidarists at work makes us as gemologists, feel very privileged to glimpse into this world.

Looking at the rough gem to determine the cut 
 

Taking the rough gem, the first step is to look at its shape. If it's not uniform or the C-Axis – where the colour is concentrated – is not in the correct orientation, the finished cut gemstone can look uneven and present windowing. This is when the gemstone "drops" its colour and appears transparent if it's tilted in a certain direction.

We use a torch to shine light though each rough and to check closely for any inclusions, while assessing the size of the stone and how these factors will impact the final gemstone. If there is a very large, dark inclusion in the centre of the rough, then it's likely we'll have to cut straight through or around it and end up with two smaller cut gems. If there is an inclusion to the side of the rough then usually, a larger, and therefore more valuable gem can be cut.

The importance of angles

The cut of a gemstone will affect how the light enters, exits and returns to your eye. Generally, the deeper the gem, the more colour saturation will be seen from a bird’s eye view, due to less light return. Think of the same effect with tissue paper – one sheet is transparent but a pack is opaque and saturated. The optimal angles for light to enter and exit a cut gemstone has been rigorously refined over the years to achieve the best resulting gem.

Gem cutting and polishing equipment

Equipment used for faceting and polishing has developed over the past 50 years. Some still use a stone and a bow driven device, known as a hanaporuwa, but many lapidarists now use a lap machine.

A lap machine consists of an electric powered plate coated in fine diamond grit, the only gem harder than sapphire, which the gemstone is lowered onto and held in a device known as a dop.

The cutting process

Once the shape has been determined by looking at the rough, the first stage in a rough gems' refinement is called pre-forming. This process uses a grinding machine that's gently pumped with water to accelerate the polishing and allow for a finer cut.

The next stages are faceting and polishing. Many people interchange these two words, however the two processes are very different. Faceting produces the symmetrical and asymmetrical faces of the gem, which is then mounted with wax onto a tool called a dop. Polishing is the final process where each facet is buffed with an ultra-fine grit to produce a smooth, reflective surface.
 

Less valuable gemstones are often cut to uniform sizes and shapes, whereas there is a little more free-range with larger and finer gemstones. The greater amount of rough that can be preserved, the higher the carat weight and therefore the price. Generally, the resulting cut gemstone is around 25-40% of the weight of the starting rough.

In our next journal entry from our trip to Sri Lanka, we venture to the cut gemstone markets in Beruwala.